1. Field of Invention
This invention relates to medical implantation for treatment of blindness. More particularly, apparatus and method are provided for treating damaged retina of the eye by implantation of dielectric-based optical micro-detectors onto the retina.
2. Description of Related Art
Damage to the retina of the eye often causes blindness. Such damage can stem from retinal atrophy or Retinitis Pigmentosa (RP). Retinitis Pigmentosa is the most prevalent cause; it is composed of a group of hereditary diseases for which no remedies are currently available. The incidence of such disease is about 0.05 per cent of the total population; however, carriers of RP genes may be as high as 2.5 per cent of the total population. RP is characterized by a progressive loss of rods in both eyes of patients, resulting in sight loss from only night blindness to total blindness. When this loss occurs, retinal neurons remain largely unaffected, according to H. Shichi, Biochemistry of Vision, (Academic Press, New York, 1983). Hence, the complex synaptic interconnections at the outer plexiform layer of the eye that would normally transmit photosignals to the nerve ganglions are intact, as are the ganglion axons or bundles which make up the optic nerve.
Previous attempts at remedying retinal blindness problems such as caused by RP have been limited to visual prosthesis which focused on direct stimulation of the brain cortex (G. S. Brinkley and W. S. Lewins, "The Sensations Produced by Electrical Stimulation of the Visual Cortex," J. of Physiology, 196, (1968); W. H. Dobelle et al, "Artificial Vision for the Blind: Electrical Stimulation of Visual Cortex Offers Hope for a Functional Prosthesis," Science 183, 1974). These attempts involved electrode stimulation of the cortex with resultant patient perception of light. It has also been proposed to replace the damaged eye with an optical "camera" and transmit electrical signals directly to the cortex (W. H. Dobelle, "Artificial Vision for the Blind," ASAIO Journal, 1994). These previous attempts have not included implantation of an optical detector onto or into the eye which would send signals through existing nerve cells to the brain.
Since in a case of RP the nerve ganglions are still intact, direct stimulation of the retinal ganglions could be decoded by the brain cortex as light intensity or color. This forms the basis for the concept of implantation of an optical detector onto the retina of a patient with retinal damage such as caused by RP. The local detector can generate a local voltage, thereby electrically exciting the neural circuit. The actual photoexcitation mechanism and resultant "seeing" is a highly complex process, as witnessed by the fact that nearly one-third of all nerve fibers entering the central nervous system are optic nerve fibers. A number of mechanisms have been invoked for optical signal encoding and transmission; however, the process is not well understood except for the agreement that local electric fields can stimulate ganglion electrical activity.
Dielectric detectors that are stable in aqueous solutions and in vivo have been developed recently (H. Lin et al, "Photoresponse and Fast Optical Readout for a PbZrTiO.sub.3 /YBa.sub.2 Cu.sub.3 O.sub.7-x Thin Film Capacitor," App. Phys. Letters, 66 (1995). Previously developed semiconductor detectors were limited by not being stable in vivo, but the recently-developed detectors are dielectrics (oxides and some nitrides) which are chemically stable in body fluids and which have been shown to exhibit optical detectivity in a thin film capacitor structure. The spectral response is governed by the optical properties of the oxide, and as a result different oxide thin films yield different spectral sensitivity.
There is a need for apparatus and method for providing an electrical field or voltage to stimulate the optical nerve fibers in the eye in response to stimulus by light. The response time of the device should be short enough to make possible effectively an instantaneous response to changes in light stimulation. Multiple sources of electrical voltage should preferably be provided so as to make possible the perception of spatial resolution by the patient. The apparatus should be unaffected by body fluids in the eye and should not be rejected by the body. Preferably, the detector should have means for anchoring on the retina so as not to move after implantation.